In U.S. Pat. No. 3,864,456 (E26), assigned to the assignee of this application, there is described a process for the production of chlorine dioxide and chlorine wherein sodium chlorate is reduced with added chloride ions in an aqueous acid reaction medium containing sulphuric acid at a low total acid normality in the range of about 2 to about 4.8 normal. The reaction medium is maintained at its boiling point under a subatmospheric pressure, so that chlorine dioxide and chlorine are removed from the reaction zone in gaseous admixture with steam. By-product anhydrous neutral sodium sulphate is deposited from the reaction medium once the reaction medium becomes saturated therewith after start-up. The gaseous mixture of chlorine dioxide, chlorine and steam removed from the reaction zone is contacted with water, usually after at least partial condensation of the steam, to form a chlorine dioxide solution also containing dissolved quantities of chlorine.
It has previously been suggested in U.S. Pat. No. 3,347,628 to form an aqueous chlorine dioxide solution from a gaseous mixture of chlorine dioxide, chlorine and steam formed from a chlorine dioxide generator to which external steam is added to dilute the gases, by contact of the gaseous mixture with water. The chlorine gas remaining from the absorption is reacted with sulphur dioxide and water to form sulphuric acid and hydrochloric acid, which are fed to the generator.
As is set forth in detail in U.S. Pat. No. 4,086,329 (E.124), assigned to the assignee of this application, the latter concept is not directly utilizable in the process of U.S. Pat. No. 3,864,456, since the chemical efficiency of chlorine dioxide production under the boiling reaction medium, subatmospheric pressure and low total acid normality conditions of the latter process is less than 100%. As described in U.S. Pat. No. 4,086,329, a critical adjustment of the hydrogen ion and chloride ion concentrations of the acid feed is required to take this inefficiency into account, otherwise continuous operation is impractical.
The reaction of chlorine, sulphur dioxide and water is described in U.S. Pat. No. 3,347,628 as being effected in a packed tower. The reaction is highly exothermic and yet no provision is made in this prior art procedure for cooling the packed tower reactor. The heat generated by the reaction elevates the temperature of the water and may readily cause evaporation of water, with consequently-incomplete dissolution of the hydrogen chloride gas into the aqueous product stream. The incomplete dissolution of the hydrogen chloride results in an improper proportion of hydrochloric acid in the acid stream recycled to the generator and the necessity for further processing of the gaseous outlet stream to remove the gaseous hydrogen chloride therefrom.
One obvious solution to this problem is to substantially increase the volume of water flowing into the reactor, so that the volume of water is sufficient also to effect cooling and prevent the aqueous medium from boiling. However, this procedure is impractical and uneconomic, in that the additional volume of water considerably dilutes the acid feed for the reactor, and may require increased reactor capacity, with consequently increased capital cost.
Dilution of the strength of the mixed acid by the increased volume of water results in this additional volume of water entering the chlorine dioxide generator and requiring a correspondingly-increased volume of water to be evaporated from the generator. The requirement to increase the volume of water evaporated in turn leads to an increased heat requirement, usually in terms of steam, since steam is the heating medium usually employed. The increased heat requirement results in substantially increased operating costs, which render the process uneconomic.
A procedure in which the by-product chlorine from the chlorine dioxide absorption is reacted with sulphur dioxide and water to form a mixture of sulphuric acid and hydrochloric acid for reuse in the generator nevertheless is a commercially-attractive one. Since hydrochloric acid is used to provide at least part of the chloride ion requirement for the chlorine dioxide-producing process, the overall amount of sodium sulphate produced per mole of chlorine dioxide produced is decreased, as is apparent from consideration of the following equations, which represent the reactions which occur when a mixture of hydrochloric acid and sulphuric acid is used. ##EQU1## wherein x is the molar proportion of HCl which is used and is a decimal value which is less than or equal to 1.00. Equation (1) represents the reaction which produces chlorine dioxide and the extent to which the reaction of equation (1) predominates over equation (2) is the efficiency of chlorine dioxide production.
It will be seen that the proportion of sodium sulphate which is produced declines as the proportion of hydrochloric acid used in place of sodium chloride and sulphuric acid increases. The requirements of pulp mills for sodium sulphate have declined while the requirements for chlorine dioxide have increased. The ability to produce less sodium sulphate for the same chlorine dioxide output through the use of hydrochloric acid, therefore, is beneficial.
Further, since chlorine gas remaining from the absorption of chlorine dioxide from the product gas stream is reacted to form reutilizable chemicals, the necessity for separate absorption of chlorine, usually in sodium hydroxide solution to form hypochlorite, is substantially decreased. With the increasing modern tendency to substitute chlorine dioxide for a substantial proportion of the chlorine which has formerly been used to effect bleaching in the first stage of a multistage bleaching operation, the requirement for chlorine has decreased, while that for chlorine dioxide has increased.
The problem to which the present invention is directed, therefore, is how to utilize the reaction of sulphur dioxide and water with chlorine from the chlorine dioxide-producing reaction and addition of that reaction product to a chlorine dioxide generating process without the prior art difficulties of loss of hydrogen chloride and increased evaporative load, and thereby achieve the benefits thereof referred to above.